As it is known, the welding of plastic pipes useful for conveying fluids uses polywelding of the head portion among the various existing technologies.
Particular welding machines are used to perform this type of weld.
Without delving into the specific details, it is important to stress that the pipes, T-shaped unions, bends et cetera that composed the pipe, in order to be welded together, must have the same diameter and thickness and must be axially aligned with each other during welding.
Particularly for the welding of pipes for pressurized fluids and in any case for pipes having a substantial thickness, welding machines are used which have a supporting frame on which four aligned vises are fitted, such vises locking in pairs the head portions of the pipes to be welded; two mutually opposite hydraulic pusher cylinders are associated with the vises and allow the movement of said vises so as to bring the heads of the pipes to be welded into contact.
In general, the vises are sized so as to clamp a specific maximum diameter and provide the possibility to apply reduction adapters.
Each vise is constituted by two jaws articulated at one end, one jaw being fixed to the frame and one being movable; a mechanical or hydraulic closure device is present at the end that lies opposite the articulation end.
Such jaws form two mutually opposite concave portions, which are substantially semicircular and form the locking seat for the corresponding pipe to be welded.
The reduction adapters of the vise consist of substantially semicircular bodies, which in practice are C-shaped and are arranged and fixed in the complementarily shaped concave portion of a corresponding jaw.
In particular, to prevent translational motions between the jaw and the reduction adapter during welding, on the former there is a slot which accommodates a rib of the latter which is shaped complementarily with respect to said slot.
As regards the fixing of the reduction adapters, nearly all known welding machines have a radial through whole at the center of the semicircular portion, in which there is a screw which engages, with its head portion, a corresponding hole formed within the jaw.
However, this system for fixing the reduction adapter to the jaw is scarcely practical and entails a certain waste of time in preparing the welding machine.
Further, because of dirt, sometimes tightening the screw becomes difficult.